Conventionally, a common rail fuel injection system calculates a target common rail pressure in accordance with an driving state of an engine, and controls an actual common rail pressure of high-pressure fuel accumulated in a common rail to be the target common rail pressure by a discharge amount of a high-pressure pump, which supplies the high-pressure fuel to the common rail, and by opening and closing a pressure-reduction valve to leak the high-pressure fuel accumulated in the common rail to a low-pressure system.
The pressure-reduction valve is for opening and closing a passage communicating an interior space of the common rail and the low-pressure system. A conventional pressure-reduction valve is a solenoid valve to open and close the passage communicating the interior space of the common rail and the low-pressure system (refer to U.S. Pat. No. 5,727,525 and its counterpart JP-09-170512-A, for example).
A control method of the conventional pressure-reduction valve is as follows.
When the target common rail pressure rapidly increases, the discharge amount of the high-pressure pump increases in accordance with the increase of the target common rail pressure, and then the actual common rail pressure increases. Here, when a detected common rail pressure detected by a common rail pressure sensor exceeds the target common rail pressure by a predetermined value, a control unit energizes the pressure-reduction valve to leak the high-pressure fuel in the common rail to prevent an overshoot of a transient pressure.
When the target common rail pressure rapidly decreases, the discharge amount of the high-pressure pump decreases in accordance with the decrease of the target common rail pressure. However, it takes some time for the actual common rail pressure to decrease to reach the target common rail pressure just by the decrease of the discharge amount of the high-pressure pump and a fuel consumption of an injector. Thus, when the target common rail pressure decreases, the control unit energizes the pressure-reduction valve to leak the high-pressure fuel in the common rail. Then, when the detected common rail pressure detected by a common rail pressure sensor decreases to be smaller than the target common rail pressure by a predetermined value, the control unit stops energizing the pressure-reduction valve to prevent an undershoot of the transient pressure (refer to JP-2002-371940-A, for example).
A specific control example of the pressure-reduction valve is described referring to FIG. 6.
When the target common rail pressure PC0 (represented by a broken line in the figure) rapidly increases, the discharge amount of the high-pressure pump increases in accordance with the increase of the target common rail pressure PC0, and the actual common rail pressure PCi (represented by a solid line in the figure) increases.
Here, when a detected common rail pressure PCk (dots in the figure) detected by the common rail pressure sensor exceeds the target common rail pressure PC0 by the predetermined value, the control unit energizes the pressure-reduction valve to leak the high-pressure fuel in the common rail. As a result, the actual common rail pressure PCi temporarily exceeds the target common rail pressure PC0 by a value larger than the predetermined value.
Further, the detected common rail pressure PCk detected by the common rail pressure sensor is detected every predetermined sampling frequency, so that sampling timings occurs detection errors in a recognition by the control unit to recognize that the detected common rail pressure PCk exceeds the target common rail pressure PC0 by the predetermined value.
That is, in the conventional art, in the case that the target common rail pressure PC0 rapidly increases, the actual common rail pressure PCi exceeds the target common rail pressure PC0 by the value larger than the predetermined value and the overshoot of the common rail pressure is relatively large, to hinder the injection control from being provided with high accuracies (for example, a poor starting operation, a generation of unusual noise, an emission deterioration and so on).
As in the case when the target common rail pressure PC0 rapidly increases, When the target common rail pressure PC0 rapidly decreases, the control unit stops a power supply to the pressure-reduction valve when the detected pressure PCk detected by the common rail pressure sensor becomes smaller than the target common rail pressure PC0 by a predetermined value, so that the actual common rail pressure PCi temporarily becomes smaller than the target common rail pressure PC0 by the predetermined value.
In a detection of a state that the detected common rail pressure PCk is smaller than the target common rail pressure PC0 by the predetermined value, a detection error occurs by the sampling timings.
That is, conventionally, when the target common rail pressure PC0 rapidly decreases, the actual common rail pressure PCi becomes smaller than the target common rail pressure PC0 by a value larger than the predetermined value, and the errors by the sampling occur. Thus, the undershoot of the common rail pressure becomes large, to hinder the injection control from being provided with high accuracies (for example, a poor starting operation, a generation of unusual noise, an emission deterioration and so on).
Further, an increase speed (increase gradient) and a decrease speed (decrease gradient) vary in accordance with a fuel temperature, so that magnitudes of the above-described overshoot and undershoot vary in accordance with the fuel temperature, and the overshoot and the undershoot are impediments to a stable injection control.